Early experiments and apparatus designed for nuclear magnetic resonance (NMR) well logging can be found in: [1] “U.S. Pat. No. 3,213,357. R. J. S. Brown, H. C. Torrey, J. Korringa, Earth formation and fluid material investigation by nuclear magnetism relaxation rate determination”; [2] “U.S. Pat. No. 4,350,955. J. A. Jackson, R. K. Cooper, Magnetic resonance apparatus”; [3] “J. A. Jackson, L. J. Burnett, J. F. Harmon, Remote (inside-out) NMR. III. Detection of nuclear magnetic resonance in a remotely produced region of homogeneous magnetic field, J. Magn. Res, 41, 1980, 411-421”; and [4] “R. L. Kleinberg, A. Sezginer, D. D. Griffin, and M. Fukuhara, Novel NMR apparatus for investigating an external sample, J. Magn. Res. 97 (1992) 466-485”.
Beginning with these early experiments and apparatus, there has been a continued interest in unilateral NMR (UMR), examples of which can be found in: [5] “J. Perlo, F. Casanova, and B. Blümich, Profiles with microscopic resolution by single-sided NMR, J. Magn. Res. 176 (2005) 64-70”; [6] “P. J. McDonald, J. Mitchell, M. Mulheron, P. S. Aptaker, J.-P. Korb, and L. Monteilhet, Two dimensional correlation relaxometry studies of cement pastes performed using a new one-sided NMR magnet, Cement and Concrete Research, In Press”; [7] “D. G. Rata, F. Casanova, J. Perlo, D. E. Demco, and B. Blümich, Self-diffusion measurements by a mobile single-sided NMR sensor with improved magnetic field gradient, J. Magn. Res, 180 (2006) 229-235”; [8] “J. Perlo, F. Casanova, and B. Blümich, Single-sided sensor for high-resolution NMR spectroscopy, J. Magn. Res., 180 (2006) 274-279”; [9] “G. Eidmann, R. Savelsberg, P. Blümler, and B. Blümich, The NMR MOUSE, a mobile universal surface explorer, J. Magn. Res. A 122 (1996) 104-109”; [10] “B. Blümich, V. Anferov, S. Anferova, M. Klein, R. Fechete, M. Adams, and F. Casanova, Simple NMR-mouse with a bar magnet, Concepts in Magnetic Resonance B 15 (2002) 255-261”; [11] “W.-H. Chang, J.-H. Chen, and L.-P. Hwang, Single-sided mobile NMR with a Halbach magnet, Magn. Reson. Imag., 24 (2006) 1095-1102”; [12] “US Patent Application 2006/0084861, A. Blank, G. Lewkonya, Y. Zur, H. Friedman, and G. Tidhar, Magnet and coil configurations for MRI probes.”; [13] “U.S. Pat. No. 5,959,454. M. Westphal, B. Knüttel, Magnet arrangement for and NMR tomography system, in particular for skin and surface examinations.”; [14] “U.S. Pat. No. 6,489,872. E. Fukushima, J. A. Jackson, Unilateral magnet having a remote uniform field region for nuclear magnetic resonance.”; [15] “A. E. Marble, I. V. Mastikhin, B. G. Coplitts, B. J. Balcom, A unilateral magnetic resonance moisture sensor for aerospace composites, in Proceedings of the Canadian Conference on Electrical and Computer Engineering, May 6-10, Ottawa, ON, Canada.”; [16] “B. Manz, A. Coy, R. Dykstra, C. D. Eccles, M. W. Hunter, B. J. Parkinson and P. T. Callaghan, A mobile one-sided NMR sensor with a homogeneous magnetic field: The NMR-MOLE, J. Magn. Res., In Press.183 (2006) 25-31”; [17] “U.S. Pat. No. 5,572,132. Y. M. Pulyer, S. Patz, MRI probe for external imaging”; [18] “S. Utsuzawa, R. Kemmer, and Y. Nakashima, Unilateral NMR system by using a novel barrel shaped magnet, Proceedings of the 5th Colloquium on Mobile NMR, Sep. 21-23, 2005, Perugia, Italy”; and [19] “J. Perlo, F. Casanova, and B. Blümich. Sensitivity analysis for single-sided sensors, Proceedings of the 6th Colloquium on Mobile NMR, Sep. 6-8, 2006, Aachen, Germany”.
UMR refers to NMR signal transduction, performed in such a way that the sample volume is external to the measurement apparatus and has the obvious advantage of allowing arbitrarily large samples to be investigated. In modern UMR hardware, permanent magnets are employed to produce the static B0 magnetic field in some remote region.
Several recent designs generate a field with a controlled spatial distribution for experiments such as profiling [5,6], diffusion [7], and spectroscopy [8]. However, most applications still rely on bulk measurements of the magnetization in a ‘sensitive volume’ defined by the inhomogeneities of B0 and B1, as discussed in: [3,4,9-17].
In the case where a sensitive volume is desired, two distinct classes of instrument exist. While many designs exist producing symmetrical 3D external sensitive volumes, for example a toroid [2], we limit the discussion here to magnets with a sensitive spot above one face. In the first class [9-12], a grossly inhomogeneous B0 field is generated by one or more magnets, and an RF coil is oriented such that B1 and B0 are orthogonal within some region. The B0 gradient along with the excitation bandwidth will define a sensitive volume. The advantages of this method include more compact magnet arrays, stronger B0 fields, and strong gradients which can sensitize measurements to slow molecular motions. Furthermore, many of these designs have B0 directed parallel to the magnet face allowing an ordinary surface coil to be used for excitation and detection, affording both simplicity and sensitivity. Drawbacks include a small spot size, and pronounced diffusive attenuation in liquid samples, both due to the high gradient. By ‘ordinary surface coil’, we mean a coil made from a simple loop of wire, generating a B1 field directed along the axis of the loop.
The second class of instrument generates a ‘sweet spot’ at which B0 contains a saddle point and is therefore locally homogeneous [3,4,13-17]. This creates a larger spot for a given excitation bandwidth; the reduced gradient limits diffusive attenuation, facilitating the measurement of liquid samples. The trade-off is that these designs generally operate at a lower field as the saddle point is obtained by field cancellation.